The present invention relates to a transparent, coextruded, multi-layer polyolefin film which is heat-sealable on both sides. The multi-layer film is characterized by high shrinkability in the longitudinal and transverse directions, in combination with excellent optical properties and good runability on high-speed wrapping machines. The invention also relates to a process for the preparation of a film of this kind and to its use.
Numerous publications including EP 194,588; Ep 8,904; and U.S. Pat. No. 4,419,111 describe biaxially oriented multi-layer polyolefin films which are heat-sealable on both sides and in which the base layer comprises propylene homopolymers and the two sealing layers, i.e., outer layers or top layers, comprise heat-sealable olefin polymers. These multi-layer polyolefin films have properties which are important for packaging films, i.e., a wide sealing range, good heat-sealability, a relatively high scratch resistance and low friction and thus favorable running characteristics on high-speed packaging machines of various types. However, with respect to optical properties, these films leave much to be desired.
The above-described films are usually prepared in such a manner that they are as dimensionally stable as possible, i.e., these films exhibit the lowest possible shrinkage in the longitudinal and transverse directions. In general, the shrink values of packaging films of this type are less than 4% in the longitudinal direction and less than 2% in the transverse direction, at a temperature of 120.degree. C.
Up to the present, it has been the general opinion that wrappings of an optically pleasing appearance can be produced from films having these low shrink values. However, the packages so prepared actually show loose or baggy wrappings with creases and wrinkles at the edges.